Extracting carrier signals from modulated signals

ABSTRACT

Methods, systems, and apparatus for detecting a center frequency of an input signal, the input signal including a carrier signal modulated with a modulation signal. Detecting a frequency of a second signal. Determining a difference signal between the center frequency of the input signal and the frequency of the second signal. Modifying the frequency of the second signal based on the difference signal to provide the carrier signal. And, outputting the carrier signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patent application Ser. No. 15/284,054, filed on Oct. 3, 2016, now U.S. Pat. No. 9,621,386, which is a continuation U.S. patent application Ser. No. 15/077,053, filed on Mar. 22, 2016, now U.S. Pat. No. 9,461,854, which is a continuation application of U.S. patent application Ser. No. 14/808,903, filed on Jul. 24, 2015, now U.S. Pat. No. 9,338,041, which are hereby incorporated by reference in their entirety.

BACKGROUND

Carrier modulation techniques are used to transmit information signals from one location to another. Traditional signal modulation techniques include, for example, amplitude modulation (AM), frequency modulation (FM), phase modulation (PM). In addition, complex modulation techniques exist that incorporate aspects of AM, FM, and PM such as quadrature phase shift keying (QPSK), amplitude phase shift keying (APSK) and including quadrature amplitude modulation (QAM).

SUMMARY

This specification relates to methods and systems for transmitting and receiving transpositional modulation (TM) signals. More specifically, the specification relates to methods and systems for extracting a carrier signal from an existing modulated signal, modulating the same carrier signal with a TM signal, and combining the existing signal with the TM signal. In addition, the specification relates to methods and systems for receiving a combined traditional modulation and TM signal on the same carrier signal and separating the TM signal from the combined signal. Although discussed in the context of TM, implementations of the present disclosure also may be applicable to transmitting and receiving signals modulated using traditional types of modulation.

In a first general aspect, innovative aspects of the subject matter described in this specification can be embodied in methods that include actions of detecting a center frequency of an input signal, the input signal including a carrier signal modulated with a modulation signal. Detecting a frequency of a second signal. Determining a difference signal between the center frequency of the input signal and the frequency of the second signal. Modifying the frequency of the second signal based on the difference signal to provide the carrier signal. And, outputting the carrier signal. Other implementations of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices.

This and other implementations can each optionally include one or more of the following features. The carrier signal can be suppressed in the input signal. The second signal can be generated by a signal generator. The signal generator can be a voltage controlled oscillator.

Modifying the frequency of the second signal can include using the difference signal to control the signal generator. The method can include limiting an amplitude of the input signal. The method can include limiting an amplitude of the second signal. The method can include amplifying the difference signal.

Detecting the center frequency of the input signal can include multiplying the input signal with a third signal to create a fourth signal, and converting the fourth signal into a direct current (DC) voltage signal representing the center frequency of the modulated signal by integrating the fourth signal. Detecting the frequency of the second signal can include multiplying the second signal with a fifth signal to create a sixth signal, and converting the sixth signal into a direct current (DC) voltage signal representing the frequency of the second signal by integrating the sixth signal.

A second general aspect can be embodied in a device that includes a signal generator, a first frequency detector, a second frequency detector electrically connected to the signal generator, the second frequency detector, and a difference circuit. The difference circuit is electrically connected to the first frequency detector, to the second frequency detector, and to the signal generator. The first frequency detector is configured to detect a center frequency of an input signal. The second frequency detector is configured to detect a frequency of an output signal of the signal generator. The difference circuit configured to determine a difference signal from an output of the first frequency detector and the second frequency detector, and to supply the difference signal to the signal generator.

This and other implementations can each optionally include one or more of the following features. The frequency of the output signal of the signal generator can be controlled by the difference signal. The device can include a first amplitude normalizing circuit electrically connected to the first frequency detector, and a second amplitude normalizing circuit electrically connected between the signal generator and the second input of the difference circuit. The first amplitude normalizing circuit can be configured to stabilize the amplitude of the input signal, and the second amplitude normalizing circuit can be to stabilize the amplitude of the output signal of the signal generator. The first amplitude normalizing circuit can be a first comparator and the second amplitude normalizing circuit can be a second comparator.

The signal generator can be a voltage controlled oscillator. The first frequency detector can output a direct current (DC) signal representing a center frequency of a modulated input signal. The second frequency detector can output a direct current (DC) signal representing a frequency of an output signal of the signal generator.

A third general aspect can be embodied in a device that includes an input terminal for receiving a modulated signal; a signal generator including a control input and an output, a first frequency detector including an input and an output; a second frequency detector including an input and an output; a difference circuit including a first input, a second input, and an output, and an output terminal electrically connected to the output of the signal generator. The input of the first frequency detector is electrically connected to the input terminal. The input of the second frequency detector is electrically connected to the output of the signal generator. The first input of the difference circuit is electrically connected to the output of the first frequency detector, the second input of the difference circuit is electrically connected to the output of the second frequency detector, and the output of the difference circuit is electrically connected to the control input of the signal generator.

In a fourth general aspect, innovative aspects of the subject matter described in this specification can be embodied in methods that include actions of receiving an input signal, where the input signal includes a carrier signal modulated with a first modulation signal and a second modulation signal, and where the second modulation signal is a TM signal. Demodulating the first modulation signal from the input signal. Modulating an un-modulated carrier signal with the first modulation signal to generate a third signal, where the third signal includes the carrier signal modulated by the first modulation signal. And, removing the first modulation signal from the input signal by subtracting the third signal from the input signal to extract the TM signal from the input signal. Other implementations of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices.

This and other implementations can each optionally include one or more of the following features. The method can include providing the TM signal to a TM demodulator. Demodulating the first modulation signal can include performing in-phase and quadrature phase demodulation. Modulating the carrier with the first modulation signal can include performing in-phase and quadrature phase modulation. The first modulation signal can be one of phase modulation, frequency modulation, binary phase shift keying, quadrature phase-shift keying, amplitude and phase-shift keying, or quadrature amplitude modulation.

The method can include filtering the first modulation signal. The method can include delaying the input signal. Demodulating the first modulation signal from the input signal can include mixing the input signal with the un-modulated carrier signal, and modulating the un-modulated carrier signal with the first modulation signal to generate a third signal can include delaying the un-modulated carrier, and modulating the delayed un-modulated carrier signal with the first modulation signal to generate the third signal.

A fifth general aspect can be embodied in a device that includes a demodulator configured to demodulate a first modulation signal from the input signal, where the input signal includes a carrier modulated with the first modulation signal and a TM signal, a modulator electrically connected to the demodulator and configured to modulate the carrier with the first modulation signal to generate a third signal that includes the carrier modulated by the first modulation signal, and a difference circuit electrically connected to the modulator and configured to remove the first modulation signal from the input signal by subtracting the third signal from the input signal.

This and other implementations can each optionally include one or more of the following features. An output of the difference circuit can be electrically connected to a TM signal demodulator. The demodulator can be an in-phase and quadrature phase demodulator. The modulator can be an in-phase and quadrature phase modulator. The first modulation signal can be one of phase modulation, frequency modulation, binary phase shift keying, quadrature phase-shift keying, amplitude and phase-shift keying, or quadrature amplitude modulation.

The device can include a filter electrically connected to the demodulator and the modulator. The device can include a delay circuit electrically connected to the difference circuit and configured to delay the input signal. The demodulator can receive an first un-modulated carrier signal as input, and the modulator can receive a second un-modulated carrier signal as input. The second un-modulated carrier signal can be delayed from the first un-modulated carrier signal.

A sixth general aspect can be embodied in methods that include actions of receiving an input signal, the input signal including a carrier signal modulated with a first modulation signal and a second modulation signal where the second modulated signal is in quadrature with respect to the first modulation signal. Demodulating the first modulation signal from the input signal. Modulating an un-modulated carrier signal with the first modulation signal to generate a third signal, where the third signal includes the carrier signal modulated by the first modulation signal. And, removing the first modulation signal from the input signal by subtracting the third signal from the input signal to extract the second modulation signal from the input signal. Other implementations of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices.

Particular implementations of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. Implementations may increase the bandwidth of signals transmitted using traditional modulation schemes. Implementations may permit the combination of two differently modulated signals on a single carrier frequency. Some implementations may permit extraction of carrier signals from a modulated signal with little or no a priori information about the modulated signal. Some implementations may be capable of extracting a carrier from a modulated signal without regard to the type of modulation used in the modulated signal. In other words, some implementations may able to extract carrier signals while being agnostic to the type modulation of an input signal.

The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example system in accordance with implementations of the present disclosure.

FIG. 2 depicts a block diagram of an example TM signal transmitter in accordance with implementations of the present disclosure.

FIG. 3A depicts a block diagram of an example carrier extractor in accordance with implementations of the present disclosure.

FIG. 3B depicts a block diagram of an example frequency detector in accordance with implementations of the present disclosure.

FIGS. 4A and 4B depict example control signals generated by a carrier signal extraction device.

FIG. 5 depicts a block diagram of an example TM signal receiver in accordance with implementations of the present disclosure.

FIG. 6A depicts a block diagram of an example TM signal separation and extraction device in accordance with implementations of the present disclosure.

FIG. 6B depicts frequency domain representations of signals at various stages of the TM signal separation and extraction device shown in FIG. 6A.

FIGS. 7 and 8 depict example processes that can be executed in accordance with implementations of the present disclosure.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

Implementations of the present disclosure generally extract a carrier signal from an existing modulated signal, modulate the extracted carrier signal with a transposition modulation (TM) signal, and combine and retransmit the existing signal with the TM signal on the same carrier signal. Specifically, the implementations can extract a carrier frequency from a modulated signal in which the carrier signal has been suppressed (e.g., QPSK, QAM, APSK, BPSK). A CAREX (carrier extraction) circuit determines a frequency difference between the frequency of the CAREX output signal and a weighted average of the carrier frequency of the input signal. The calculated difference value is used to continuously tune a signal generator to maintain a minimal difference between the weighted average of the input carrier frequency and the CAREX output. The third harmonic of the extracted carrier is modulated with a data signal generating a TM modulated signal. The TM modulated signal is heterodyned back to the extracted carrier frequency and combined with the existing modulated signal. The combined signal can then be transmitted. Moreover, the TM modulated signal in the combined signal does not interfere with the existing signal because the TM modulation is not recognized by demodulation systems used to demodulate traditional modulation schemes. Instead, the TM signal appears as a slight increase in noise within the existing signal.

Other implementations of the present disclosure generally receive a combined traditional modulation and TM signal on the same carrier signal then separate the TM signal from the combined signal. Specifically, the implementations can separate the existing signal from a combined signal including a traditionally modulated signal (the existing signal) and a TM modulated signal. The existing signal can be demodulated from the carrier signal. An extracted carrier signal can be re-modulated with the demodulated existing signal to re-create the existing signal alone, absent the TM modulated signal. The re-modulated existing signal can be removed from the combined signal leaving only the TM modulated signal which can be demodulated using TM demodulation techniques described herein.

FIG. 1 depicts an example system 100 in accordance with implementations of the present disclosure. The system 100 is a system of signal transmitters and receivers 102. The system 100 may be a radio frequency (RF) communication system, a satellite communication system, a landline communication system (e.g., a telephony or cable network), an optical communication system, or any other system of signal transmitters and receivers 102. The transmitters/receivers 102 include systems for modulating a carrier signal with an information signal using traditional modulation techniques and transmitting the modulated signal from one transmitter/receiver to another. Traditional modulation techniques include, for example, amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM) in addition to complex modulation techniques that incorporate aspects of AM, FM, and PM such as quadrature phase shift keying (QPSK), amplitude phase shift keying (APSK) and including quadrature amplitude modulation (QAM). In addition, transmitter/receiver B includes a TM transmitter 104 and transmitter/receiver C includes a TM receiver 106.

The system 100 can receive a traditionally modulated signal 108 and combine the traditionally modulated signal 108 with a TM modulated signal 110 on the same carrier using a TM transmitter 104, thereby increasing the overall bandwidth of the combined signal 112. The TM modulated signal 110 can be separated from the combined signal 112 and demodulated by a TM receiver 106. Likewise, the traditionally modulated signal 108 can be separately demodulated with no interference caused by the TM modulated signal 110. This is possible because TM modulated signals are undecipherable by non-TM receivers, instead appearing as a slight increase of noise in traditionally modulated signals.

For example, transmitter/receiver A may transmit a QAM signal 108 to transmitter/receiver B. The TM transmitter 104 at transmitter/receiver B can receive the QAM signal 108 and extract the carrier signal from the QAM signal 108. The TM transmitter 104 modulates the extracted carrier signal with a TM signal, combines the TM modulated signal 110 with the QAM signal 108, and retransmits the combined signal 112. In some examples, as described below, the TM transmitter 104 can extract a carrier signal from a traditionally modulated signal 108 (e.g., the QAM signal) in which the carrier is suppressed and while having little or no a priori information about the carrier signal (e.g., frequency or phase information).

Transmitter/receiver C can then receive the combined signal 112. The TM receiver 106 of transmitter/receiver C separates and extracts the TM modulated signal 110 from the combined signal 112, and then demodulates the TM modulated signal 110 to obtain the TM modulated data signal. In some examples, as described below, the TM receiver 106 separates the TM modulated signal 110 from the combined signal 112 by demodulating traditionally modulated signal 108 (e.g., the QAM signal), re-modulating the carrier with only the traditionally modulated signal 108, and subtracting the re-modulated carrier signal from the combined signal 112 leaving only the TM modulated signal 110.

In some implementations, the carrier signal can be an intermediate frequency (IF) carrier signal. That is, the carrier signal is not necessarily at the same frequency of the carrier upon which the signal is ultimately be transmitted, but may be at an IF used internally within a system (e.g., a satellite communication system) as an intermediate step in either signal transmission or reception. That is, in the case of signal transmission, a system may up-convert a combined signal 112 from the IF signal to a transmission carrier frequency prior to transmitting the combined signal 112. Conversely, in the case of signal reception, a system may down-convert a modulated signal from the transmission carrier frequency to an IF frequency before separating the TM modulated signal 110 from the combined signal 112. In other implementations, an IF carrier signal may not be used, and the transmission carrier signal can be modulated with both a traditionally modulated signal and a TM modulated signal.

FIG. 2 depicts a block diagram of an example TM signal transmitter 104 in accordance with implementations of the present disclosure. The TM transmitter 104 includes a carrier extraction portion (CAREX 206), a harmonic generation portion 202, a TM modulating portion 204, and a heterodyning portion 205. The carrier extraction portion includes the carrier extractor (CAREX) 206. The harmonic generation portion 202 includes a second harmonic generator 208 and a third harmonic generator 210. The TM modulating portion 204 includes a signal optimizer 212 and a TM modulator 214. And, the heterodyning portion 205 includes a signal mixer 216, a bandpass filter 218, and a power amplifier 220. In addition, the TM transmitter 104 includes a signal coupler 222 and a signal combiner 224.

In operation, the TM transmitter 104 receives an existing modulated signal (e.g., traditionally modulated signal 108 of FIG. 1). The signal coupler 222 samples the existing modulated signal and passes the sample of the existing modulated signal to the CAREX 206. The CAREX 206 extracts a carrier signal (f_(c)) from the existing modulated signal. The CAREX 206 is described in more detail below in reference to FIGS. 3A-4B. The output of the CAREX 206 is a pure sinusoidal signal at the fundamental frequency of the carrier from the existing modulated signal. In some examples, the CAREX 206 is agnostic to the type of modulation used in the existing modulated signal. That is, the CAREX 206 can extract the carrier signal from an existing modulated signal regardless of the type of modulation used in the existing modulated signal. In some examples, the CAREX 206 can extract carrier signals even when the carrier is suppressed in the existing modulated signal, and can do so with little or no a priori information about existing modulated signal's carrier (e.g., frequency or phase modulation information).

The CAREX 206 passes the extracted carrier signal to a second harmonic signal generator 208 and a third harmonic signal generator 210, which generate signals at the second and third harmonic frequencies (2f_(c) and 3f_(c) respectively) of the fundamental carrier frequency (f_(c)). The second and third harmonic signals (2f_(c), 3f_(c)) are used by the TM modulation portion 204 and the heterodyning portion 205 of the TM transmitter 104 to generate a TM modulated signal and to heterodyne the TM modulated signal to the fundamental carrier frequency (f_(c)).

The TM modulation portion 204 of the TM transmitter 104 modulates the third harmonic (3f_(c)) of the carrier signal (f_(c)) with a data signal to generate the TM modulated signal. The TM modulated signal is then heterodyned to the frequency of the carrier signal (f_(c)), combined with the existing modulated signal, and outputted to an antenna for transmission.

In more detail, TM modulation portion 204 receives a data signal for transmission (e.g., a baseband (BB) data signal). The BB data signal can be, for example, an analog signal that may be quantized to convey digital bits. The data signal is optionally processed for transmission as a TM modulated signal by the signal optimizer 212. In some examples, the signal optimizer 212 produces an optional pattern of inversion and non-inversion of the modulating signal, and filters the modulating signal to ensure that the total bandwidth of the data signal is within the channel bandwidth of the existing modulated signal. In some examples, the signal optimizer 212 can include sample-and-hold circuitry and filters to prepare the modulating signal for transmission as a TM modulated signal. In some examples, the signal optimizer 212 can be bypassed or turned off and on.

The TM modulator 214 modulates the third harmonic (3f_(c)) of the carrier signal (f_(c)) with a data signal to generate the TM modulated signal. For example, the TM modulator 214 modulates the third harmonic (3f_(c)) by introducing a variable time delay based on the data signal. In other words, the TM modulator 214 can use the data signal as a control signal for introducing an appropriate time delay to third harmonic (3f_(c)). As such, an amount of time delay introduced into the third harmonic (3f_(c)) represents discrete bits or symbols of the data signal. The described time delay modulation technique may be considered as time-shift modulation and is performed on the third harmonic (3f_(c)) of the intended carrier frequency (3f_(c)).

The time-shift modulation of the third harmonic (3f_(c)) produces a single set of upper and lower Bessel function sidebands. The inventor has confirmed such results in laboratory simulations with an oscilloscope and spectrum analyzer. Moreover, the bandwidth of these sidebands can be limited to the bandwidth of an intended communication channel by the optimizer 212 before TM modulation of the signal, as described above.

In some examples, the time delay may be a phase shift. However, the time-shift modulation described above is not equivalent phase modulation. As noted above, the inventor has confirmed in laboratory tests that the time-shift modulation only produces a single pair of upper and lower Bessel sidebands. Phase modulation, however, produces a series upper and lower Bessel sidebands.

The heterodyning portion 205 prepares the TM modulation signal do be combined with the existing modulated signal and transmitted by the receiver. The TM modulated signal is then heterodyned (e.g., frequency shifted) by mixer 216 down to the fundamental frequency of the carrier signal (f_(c)). The mixer 216 multiplies the TM modulated signal with the second harmonic of the carrier (2f_(c)) which shifts the TM modulated signal to both the fundamental carrier signal frequency (f_(c)) and the fifth harmonic frequency of the carrier. The bandpass filter 218 removes signal at the fifth harmonic frequency as well as any additional signals or noise outside of the bandwidth of the TM modulated signal centered at the fundamental carrier signal frequency (f_(c)).

The TM modulated carrier signal is amplified by power amplifier 220 and combined with the existing modulated signal by the signal combiner 224. It may be necessary, in some examples, to adjust the phase of the TM modulated carrier signal to match the phase of the carrier in the existing modulated signal before combining the two signals.

FIG. 3A depicts a block diagram of an example CAREX 206 in accordance with implementations of the present disclosure. The CAREX 206 can be implemented as a circuit in a device such as a TM transmitter or TM receiver, for example. In some implementations, the CAREX 206 can be implemented as a standalone device for installation into in a larger system (e.g., an application specific integrated circuit (ASIC) or field programmable logic array (FPGA)). In some implementations, the CAREX 206 can be implemented in software, for example, as a set of instructions in a computing device or a digital signal processor (DSP).

The CAREX 206 operates by determining a center frequency of an input signal (e.g., either modulated or unmodulated), comparing the center frequency to the frequency of a pure sinusoidal signal produced by a signal generator to create an error signal, and adjusting the frequency of the signal generator output signal based on a control signal generated from the error signal until the error signal is minimized. Furthermore, the CAREX 206 does not require a priori information about a carrier signal to extract the carrier signal and can extract carrier signals when the carrier of the modulated signal is suppressed.

The CAREX 206 includes amplitude limiters 302 a, 302 b, filters 304 a, 304 b, frequency detectors 306 a, 306 b, signal generator 308, difference circuit 310, and an amplifier 312. The amplitude limiter 302 a and filter 304 a condition input signal before the input signal is analyzed by the first frequency detector 306 a. The amplitude limiter 302 a removes any variations in the amplitude of the input signal. In other words, the amplitude limiter 302 a stabilizes the amplitude of the input signal. In some examples, the amplitude limiters 302 a, 302 b can be an analog comparator or an automatic gain control (AGC) circuit. The filters 304 a, 304 b are bandpass filters and removes extraneous signals (e.g., harmonics) and noise outside the channel bandwidth of the input signal.

The frequency detectors 306 a and 306 b can be frequency discriminators or quadrature detectors. The first frequency detector 306 a detects the center frequency of the input signal. As shown in the frequency domain plot 320, an input signal produced by traditional modulation techniques generally has symmetric sidebands 322 located on either side of the carrier frequency 324. The frequency detector 306 a can determine a center frequency of an input signal based on, for example, the frequencies of the outer edges of the sidebands 322. Furthermore, the frequency detector 306 a can use the sidebands 322 of an input signal to determine the center frequency even if the carrier signal 324 is suppressed, as illustrated by the dotted line.

The signal generator 308 generates a pure sinusoidal signal (e.g., a single frequency signal) which is provided to a second frequency detector 306 b. The signal generator 308 can be, for example, a voltage controlled oscillator (VCO) such as, but not limited to, a voltage controlled LC (inductor-capacitor) oscillator circuit, a voltage controlled crystal oscillator (VCXO), or a temperature-compensated VCXO. The second frequency detector 306 b detects the frequency of the output signal from the signal generator 308. In some examples, the output signal from the signal generator 308 is provided to an amplitude limiter 302 b and filter 304 b before being transmitted to the second frequency detector 306 b. The amplitude limiter 302 b and filter 304 b stabilize and filter the amplitude of the signal generator output signal similar to amplitude limiter 302 a and filter 304 a.

The output from each of the first and second frequency detectors 306 a, 306 b is provided as inputs to the differencing circuit 310. The output of both the first and second frequency detectors 306 a, 306 b can be, in some examples, a direct current (DC) voltage signal representing the center frequency of the input signal and the frequency of the signal generator 308 output signal, respectively. The output of the difference circuit 310 is an error signal representing the difference in frequency between the center frequency of the input signal in the signal generator output signal. The error signal (e.g., a DC voltage) is amplified by amplifier 312 and provided as a control signal to the signal generator 308. The amplifier 312 can be, for example, a high gain integrating circuit that integrates the inputted error signal over time to produce the control signal.

The signal generator 308 adjusts the frequency of its output signal based on the control signal until the frequency of the signal generator 308 output is matched to the center frequency of the input signal. The DC value of the control signal is used to control the frequency of the signal generator output, as shown in FIG. 4B and described below. The signal generator output is provided as the output of the CAREX 206. Frequency domain plot 330 and time domain plot 334 represent an example CAREX 206 output signal. As shown, the output signal of the CAREX 206 is a pure sinusoidal signal having a frequency 332 equivalent to the fundamental carrier frequency of the input signal.

In some implementations, the frequency detectors 306 a and 306 b are matched. In some examples, the matched frequency detectors 306 a and 306 b have similar frequency to DC output characteristics over changing modulated input frequencies. In some examples, the matched frequency detectors 306 a and 306 b have similar thermal and aging properties. In some examples, the amplitude limiters 302 a and 302 b, and the filters 304 a and 304 b are matched.

In some examples, when the error signal is minimized the signal generator output is effectively matched to the center frequency of the input signal. For example, the error signal can be considered as minimized when its magnitude is zero or substantially close to zero (e.g., when the control signal has a magnitude that is negligible in relation signal magnitudes measureable or usable by components of the CAREX 206). In some examples, the error signal is considered to be minimized when its magnitude is below a threshold value (e.g., an error tolerance threshold).

In some implementations, the CAREX 206 is adapted to extract carrier frequencies from single sideband signals. In some examples, the CAREX 206 includes a controller that offsets the output signal of the signal generator 308 by an appropriate offset frequency. For example, the output of the frequency generator 308 can be offset after it is fed back to the second frequency detector 306 b, so as to not adversely affect the control signal. In some examples, the first frequency detector 306 a can be configured to determine a frequency offset based on the bandwidth of the input signal. In such examples, the first frequency detector 306 a can adjust the detected frequency by the frequency offset.

FIG. 3B is a block diagram of an example frequency detector 306 in accordance with implementations of the present disclosure. The frequency detector 306 illustrated in FIG. 3B is an example quadrature-based detector circuit. The frequency detector 306 includes a phase shift network 350, a signal mixer 352, and a filter 354. The phase shift network 350 is a frequency sensitive circuit, such as an all pass filter, for example, that causes a phase shift in an input signal that corresponds with the frequency of the input signal. In other words, the phase shift network 350 causes a change in the phase angle of the input signal relative to the frequency of the input signal. In some examples, the phase shift network 350 is tuned to produce a nominal phase shift of 90 degrees (e.g., quadrature to the input signal) for a nominal design frequency (e.g., a 70 MHz IF for a communication system).

The signal mixer 352 can be, for example, a signal multiplier. The signal mixer 352 receives the input signal and an output signal from the phase shift network 350 as inputs. The filter 354 is a low pass filter.

Plot 360 shows example signals at various points in the frequency detector 306. The input signal (Signal A) is passed to the phase shift network 350 and the signal mixer 352. Signal A is shown as a sinusoid for simplicity, however, Signal A can be a modulated signal. Signal B is the output of the phase shift network 350 and is phase shifted relative to the input signal (Signal A). The value of the phase shift corresponds to the frequency of Signal A, and is nominally 90 degrees for a design frequency. Deviations from the design frequency resulting in a phase shift of Signal B that deviates from the nominal 90 degrees. The input signal (Signal A) is mixed with the output of the phase shift network 350 (Signal B) to produce Signal C (e.g., Signal C=Signal A×Signal B). Signal C has a DC offset component corresponding to the phase difference between Signals A and B, and by extension, to the frequency of Signal A. The low pass filter 354 then removes the high frequency components of Signal C leaving only the DC component (Signal D). The deviation of Signal B's phase shift from the nominal 90 degrees is exaggerated in plot 360 in order to clearly show the resulting DC output signal (Signal D).

FIG. 4A depicts a plot 400 of an example control signal 402 generated in an example CAREX 206. The plotted control signal 402 is an example of the input signal to the signal generator 308 of FIG. 3A. The plotted control signal 452 is broken into several regions (406-410). The regions illustrate a variations 404 in the control signal 402 as the input signal to the CAREX 206 is switched between several different input signals, each modulated using a different type of modulation. The input signal in region 406 is a QPSK modulated signal. The input signal in region 408 is a QAM modulated signal. The input signal in region 410 is an unmodulated carrier signal. Each of the input signals in regions 406-410 is applied to a 70 MHz carrier. The plot 400 illustrates the robustness of the CAREX 206 and its adaptability to extracting carrier signals from various input signals without regard to the types of modulation applied to the carrier signal.

FIG. 4B depicts a plot 450 of another example control signal 452 generated in an example CAREX 206. The plotted control signal 452 is an example of the input signal to the signal generator 308 of FIG. 3A. The plotted control signal 452 is broken into several regions (456-460). The regions illustrate transitions 454 of the control signal 452 as the input signal to the CAREX 206 is switched between several different input signals, each having a different carrier frequency. The input signal in region 456 is a 67 MHz carrier signal. The input signal in region 458 is a 73 MHz carrier signal. The input signal in region 460 is a 70 MHz carrier signal. The plot 450 illustrates the robustness of the CAREX 206 and its adaptability to extracting different frequency carrier signals. In some implementations, as shown, the CAREX 206 loop can be designed for a specific center frequency (e.g., 70 MHz as shown). For example, the design center frequency can be a specific carrier frequency or IF of a communication system such as a satellite or radio frequency (RF) communication system, for example.

FIG. 5 depicts a block diagram of an example TM signal receiver 106 in accordance with implementations of the present disclosure. The TM receiver 106 includes a carrier extraction portion (e.g., CAREX 506), a harmonic generation portion 504, a signal separation and extraction portion (SEPEX) device 512, and a TM demodulator 514. As in the TM transmitter 104, the harmonic generation portion includes a second harmonic generator 508 and a third harmonic generator 510. In addition, the TM receiver 106 can include a signal splitter 502 to split a combined input signal (e.g. combined signal 112 of FIG. 1) between the TM receiver 106 and a signal receiver for traditional modulated signals.

In operation, the TM receiver 106 receives a combined input signal and provides the combined signal to both the CAREX 506 and SEPEX device 512. As described above in reference to the TM receiver 106, the CAREX 506 extracts a carrier signal (f_(c)) from the combined signal, and the second harmonic generator 508 and third harmonic generator 510, respectively, generate second and third harmonics (2f_(c) and 3f_(c)) of the extracted fundamental carrier frequency (f_(c)). Both the carrier signal (f_(c)) and second harmonic signal (2f_(c)) are provided to the SEPEX device 512. The third harmonic signal (3f_(c)) is provided to the TM demodulator 514.

The TM demodulation portion 504 separates and extracts the traditionally modulated signal from the combined signal to obtain the TM modulated signal. The SEPEX device 512 provides the TM modulated signal to the TM demodulator 514, which, demodulates the TM modulated signal to obtain a baseband data signal. The SEPEX device 512 separates and extracts the TM modulated signal from the combined signal. In some implementations, before outputting the TM modulated signal, the SEPEX device 512 heterodynes (e.g., up-shifts) the TM modulated signal to the third harmonic frequency (3f_(c)) for demodulation. The SEPEX device 512 is described in more detail below in reference to FIG. 6A.

The TM demodulator 514 uses the third harmonic signal (3f_(c)) provided by the third harmonic generator 210 as a reference signal for TM demodulation. The TM demodulator 514 demodulates the TM signal by sensing the time shifts between TM modulated carrier signal from the SEPEX device 512 and the third harmonic signal (3f_(c)). In some examples, the TM demodulator 514 can be a phase detection circuit. In some implementations, the TM demodulator 514 detects the time shifts by determining a correlation between the TM modulated carrier signal and the third harmonic signal (3f_(c)) based on, for example, a product of the two signals.

FIG. 6A depicts a block diagram of an example TM signal SEPEX device 512 in accordance with implementations of the present disclosure. The SEPEX device 512 can be implemented as a circuit in a device such as a TM receiver, for example. In some implementations, the SEPEX device 512 can be implemented as a standalone device for installation into in a larger system (e.g., an application specific integrated circuit (ASIC) or field programmable logic array (FPGA)). In some implementations, the SEPEX device 512 can be implemented in software, for example, as a set of instructions in a computing device or a digital signal processor (DSP).

In operation, the SEPEX device 512 demodulates the traditionally modulated signal from the combined signal. Because the TM modulation is not detected by traditional signal demodulation, the resulting signal does not include the TM signal, but only the demodulated data signal from the traditional modulation signal. A “clean” (e.g., un-modulated) carrier is then re-modulated with the previously demodulated data signal from the traditional modulation signal. The SEPEX 512 computes the difference between the combined signal and the re-modulated signal to obtain a TM modulated carrier signal. In other words, the SEPEX device 512 removes a traditionally modulated signal from the combined signal by demodulating the traditionally modulated signal, re-modulating a “clean” (e.g., un-modulated) carrier, and subtracting the re-modulated signal from the combined signal, thereby, leaving only the TM modulated carrier.

The SEPEX device 512 includes a signal demodulator 602, a signal modulator 604, low-pass filters 606 a, 606 b, a summing circuit 608, a difference circuit 610, a delay circuit 612, a mixer 614, a bandpass filter 616, and an amplitude limiter 618. The demodulator 602 is a non-TM signal demodulator, and the modulator 604 is a non-TM signal modulator. That is, the demodulator 602 and modulator 604 are traditional modulation type (e.g., AM, FM, PM, QAM, APSK, etc.) demodulator and modulator. The demodulator 602 and modulator 604 are depicted as a complex (e.g., quadrature and in-phase) demodulator and modulator, however, in some examples the demodulator 602 and modulator 604 can be a simple (e.g., single phase) demodulator and modulator.

The operation the SEPEX device 512 is described below in more detail and with reference to FIGS. 6A and 6B. FIG. 6B depicts frequency domain representations of signals (A-F) at various stages of the SEPEX device 512. The demodulator 602 receives the combined signal (A) (e.g. combined signal 112 of FIG. 1) as one input, and the carrier signal (f_(c)) from the CAREX 506 as a second input. The combined signal includes both a traditionally modulated signal and a TM modulated signal. As shown by signal (A) in FIG. 6B, the combined signal includes frequency content from both the TM modulated signal and the traditionally modulated signal centered about the carrier frequency (f_(c)). The demodulator 602 demodulates the traditional modulated signal from the combined signal producing a baseband data signal. As noted above, because the TM modulation is not detected by traditional signal demodulation, the resulting baseband data signal does not include a TM signal.

In the case of complex modulation, the demodulator 602 demodulates both the in-phase and quadrature phase of the combined signal producing an in-phase and a quadrature phase baseband data signal. The low-pass filters 606 a and 606 b remove any extraneous signals or noise from the baseband data signals, for example, harmonics introduced by the demodulation process. The resulting baseband data signal, shown by signal (B), includes only the frequency content from the traditionally modulated signal centered at zero frequency (baseband). More specifically, a TM modulated signal does not exist at baseband, and thus, the TM modulated signal is removed by converting the traditionally modulated signal to baseband.

The modulator 604 receives the baseband data signals (e.g., in-phase and quadrature phase signals) as a first input, and the carrier signal (f_(c)) from the CAREX 506 as a second input. The modulator 604 re-modulates the un-modulated carrier signal (f_(c)) from the CAREX 506 with the baseband data signals resulting in re-modulated carriers (re-modulated in-phase and quadrature phase carriers) having only the traditionally modulated signal. The in-phase and quadrature phase re-modulated carriers are combined by the summing circuit 608 (signal (C)). FIG. 6B signal (C) shows the re-modulated signal again centered about the carrier frequency (f_(c)). In some examples, the carrier signal (f_(c)) may be phase shifted or delayed to account for delays introduced into the baseband data signals during the demodulation and filtering process. This is to ensure that the resulting re-modulated signal is in phase with the combined signal.

The re-modulated signal is subtracted from the combined signal by the difference circuit 610 removing the traditionally modulated signal from the combined signal. The resulting signal, show by signal (D), includes only the TM modulated carrier signal (f_(c)). The combined signal is delayed by the delay circuit 612 to account for delays introduced into the re-modulated signal by the demodulation and re-modulation process.

The TM modulated signal is heterodyned (e.g., up-shifted) to the third harmonic (3fc) by the mixer 614. The mixer 614 multiplies the TM modulated signal with the second harmonic (2f_(c)) of the carrier from the second harmonic generator 508 producing signal (E). Heterodyning the TM modulated carrier signal (f_(c)) with the second harmonic (2fc) shifts the TM modulated signal to both the third harmonic (3fc) and the negative carrier frequency (−fc) (e.g., a phase inverted version of the TM modulated signal at the carrier frequency). The bandpass filter 616 removes the phase inverted TM signal at the carrier frequency leaving only the TM modulated third harmonic (3fc) (signal (F)), and the optional amplitude limiter 618 removes any variations in the amplitude of the TM modulated third harmonic signal.

In some examples, the SEPEX device 512 can include multiple different types of demodulators 602 and modulators 604. For example, the SEPEX device 512 can include FM, PM, and QAM demodulators 602 and modulators 604. In such examples, the SEPEX device 512 can also include a control device that detects the type of traditional modulation on input signal, and sends the input signal to the appropriate set of demodulator and modulator.

Although the SEPEX device 512 is described in the context of separating and extracting a TM modulated signal from a traditionally modulated signal, in some implementations, the SEPEX device 512 can be modified to separate two traditionally modulated signals such as separating non-quadrature modulated signals (e.g., in-phase modulated signal) and quadrature modulated signals. For example, a non-quadrature modulated signal could be separated and extracted from a combined I/Q modulated signal by modifying the SEPEX device 512 shown in FIG. 6A such that only the quadrature modulated signal is demodulated and demodulated by demodulator 602 and modulator 604.

FIG. 7 depicts an example process 700 for extracting a carrier frequency from an input signal that can be executed in accordance with implementations of the present disclosure. In some examples, the example process 700 can be provided as computer-executable instructions executed using one or more processing devices (e.g., a digital signal processor) or computing devices. In some examples, the process 700 may be hardwired electrical circuitry, for example, as an ASIC or an FPGA device.

A center frequency of an input signal is detected (702). For example, the center frequency can be detected based on frequency side lobes of the input signal. In some examples, the input signal can include the carrier signal modulated with the modulation signal. In some examples, the input signal is a carrier signal modulated with a traditional modulation signal and a TM modulation signal. A frequency of a second signal is detected (704). For example, the second signal may be the output of a single generator such as, for example, a VCO or a VCXO. A difference signal (e.g., control signal) is determined based on the center frequency of the input signal and the frequency of the second signal (706). For example, the difference signal represents a difference in frequency between the center frequency of the input signal and the frequency of the second signal. In some examples, difference signal is a DC voltage signal.

The frequency of the second signal is modified based on the difference signal to provide the carrier signal of the input signal (708), and the second signal is outputted as the carrier signal from the device performing the process 700 (710). For example, a difference signal can be a control signal for the signal generator and can cause the single generator to adjust the frequency of its output signal. The frequency of the second signal modified until it is matched to the center frequency of the input signal. In some examples, the frequency of the second signal is matched to the center frequency of the input signal when the difference signal reaches a minimum value. In some examples, the minimum value may be a threshold value indicating that the difference between the frequency of the second signal in the center frequency of input signal is within an allowable tolerance. In some examples, the minimum value may be a magnitude of the different signal voltage that is below the threshold minimum voltage magnitude.

FIG. 8 depicts an example process 800 for separating TM signals from input signals that can be executed in accordance with implementations of the present disclosure. In some examples, the example process 800 can be provided as computer-executable instructions executed using one or more processing devices (e.g., a digital signal processor) or computing devices. In some examples, the process 800 may be hardwired electrical circuitry, for example, as an ASIC or an FPGA device.

An input signal including a carrier signal modulated with a first modulation signal and a second modulation signal is received (802). For example, the first modulation signal may be a traditional type of modulation signal such as, for example, FM, AM, PM, QAM, APSK, etc. The second modulation signal may be a TM modulation signal. The first modulation signal is demodulated from the input signal (804). For example, the first modulation signal can be demodulated using traditional the modulation techniques. Because traditional demodulation techniques do not recognize TM modulation, the resulting demodulated first modulation signal will not include the TM modulation signal.

The carrier signal is re-modulated using the demodulated first modulation signal to generate a third signal (806). For example, the third signal includes an un-modulated carrier signal modulated with the first modulation signal. The un-modulated carrier signal has the same frequency as the carrier of the input signal. The first modulation signal is removed from the input signal by subtracting the third signal from the input signal (808) to extract the second modulation signal (e.g., the TM modulation signal) from the input signal. In some examples, the input signal must be delayed an appropriate amount of time to ensure that it is in phase with the third signal. That is, due to the demodulation and re-modulation process the third signal may be out of phase with the original input signal. Thus, before subtracting the third signal from the input signal, the input signal can be delayed an appropriate amount of time. The extracted second modulation signal is provided to a signal demodulator (810). For example, an extracted TM modulated signal can be provided to a TM signal demodulator for demodulation.

Implementations of the subject matter and the operations described in this specification can be realized in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Implementations of the subject matter described in this specification can be realized using one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal; a computer storage medium can be a source or destination of computer program instructions encoded in an artificially generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices).

The operations described in this specification can be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources.

The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing. The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures.

A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program can, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network.

The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit).

Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. Elements of a computer can include a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any implementation of the present disclosure or of what can be claimed, but rather as descriptions of features specific to example implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features can be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination can be directed to a sub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing can be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products.

Thus, particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing can be advantageous. 

What is claimed is:
 1. A signal transmitter comprising: at least one processor; and a data store coupled to the at least one processor having instructions stored thereon which, when executed by the at least one processor, causes the at least one processor to perform operations comprising: receiving an input signal, the input signal comprising a carrier signal modulated with a non-transpositional modulation (non-TM) signal; detecting a center frequency of the input signal; detecting a frequency of a second signal; determining a difference between the center frequency of the input signal and the frequency of the second signal; modifying the frequency of the second signal based on the difference between the center frequency of the input signal and the frequency of the second signal until the second signal has a fundamental frequency of the carrier signal; providing the second signal to a transpositional modulation (TM) modulator; generating a TM signal using the second signal; and combining the TM signal with the input signal to provide a combined signal that includes the carrier signal modulated with the non-TM signal and the TM signal.
 2. The transmitter of claim 1, wherein the at least one processor includes one of: a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or a digital signal processor (DSP).
 3. The transmitter of claim 1, wherein the carrier signal is suppressed in the input signal.
 4. The transmitter of claim 1, wherein the second signal is generated by a signal generator.
 5. The transmitter of claim 4, wherein the signal generator is a voltage controlled oscillator coupled to the at least one processor.
 6. The transmitter of claim 4, wherein modifying the frequency of the second signal comprises controlling the signal generator based on the difference between the center frequency of the input signal and the frequency of the second signal.
 7. The transmitter of claim 1, wherein the operations comprise: limiting an amplitude of the input signal; and limiting an amplitude of the second signal.
 8. The transmitter of claim 1, wherein the method further comprises amplifying the difference signal.
 9. The transmitter of claim 1, wherein detecting the center frequency of the input signal comprises: multiplying the input signal with a third signal to create a fourth signal; and converting the fourth signal into a direct current (DC) voltage signal representing the center frequency of the input signal by integrating the fourth signal.
 10. The transmitter of claim 9, wherein the third signal is a phase shifted version of the input signal.
 11. The transmitter of claim 1, wherein detecting the frequency of the second signal comprises: multiplying the second signal with a third signal to create a fourth signal; and converting the fourth signal into a direct current (DC) voltage signal representing the frequency of the second signal by integrating the fourth signal.
 12. The transmitter of claim 11, wherein the third signal is a phase shifted version of the second signal.
 13. The transmitter of claim 1, wherein the operations further comprise: receiving a second input signal having a different center frequency from the input signal; and modifying the frequency of the second signal based on a difference between the different center frequency of the second input signal and the frequency of the second signal until the second signal represents the frequency of a carrier signal of the second input signal.
 14. The transmitter of claim 1, wherein generating the TM signal based on the second signal comprises modulating a harmonic of the second signal to produce the TM signal.
 15. The transmitter of claim 1, wherein generating the TM signal based on the second signal comprises time-shifting a harmonic of the second signal to represent data of a data signal.
 16. The transmitter of claim 15, wherein the harmonic of the second signal is a third harmonic.
 17. The transmitter of claim 16, wherein the operations further comprise before combining the TM signal with the input signal, frequency shifting the TM signal to the fundamental frequency of the carrier signal by mixing the TM signal with a second harmonic of the second signal.
 18. The transmitter of claim 1, wherein the operations further comprise before combining the TM signal with the input signal, frequency shifting the TM signal to the fundamental frequency of the carrier signal.
 19. The transmitter of claim 1, wherein the non-TM signal is one of phase modulation, frequency modulation, binary phase-shift keying, quadrature phase-shift keying, amplitude shift keying, or quadrature amplitude modulation.
 20. The transmitter of claim 1, wherein generating the TM signal based on the second signal comprises modulating the second signal to produce the TM signal by: generating a second harmonic signal of the second signal and a third harmonic signal of the second signal; modulating the third harmonic signal based on a data signal to provide a modulated third harmonic signal; and mixing the modulated third harmonic signal with the second harmonic signal to provide the TM signal at the fundamental frequency of the carrier signal. 